The Globular Clusters and Halo of M31

Abstract

This thesis presents the results of a study of the globular cluster (GC) system in the Andromeda galaxy (5M31 5 NGC 224). Deep Vand I-band images of 24 M31 GCs were taken with the High Resolution Camera (HRCam) at the CanadaFrance-Hawaii Telescope (CFHT) with typical seeing full widths at half-maxima of . These images are the FWHM ∼ 00.7 best ground-based data to date for determining structural parameters (such as core and tidal radii, ellipticities, and orientations) of M31’s GCs. Two-dimensional seeing-convolved Michie-King models were fitted to each GC, and the results were compared to the structures of the Galactic GCs. There is no evidence for any differences between the mean tidal radii and ellipticities of the GCs in the two galaxies. Core radii and half-mass radii are too strongly affected by seeing to allow such a comparison to be made with these images. Deep ( ) Vand I-band images of fields containing V ∼ 27 lim G302 and G312, two GCs in the M31 halo, were obtained using the Wide Field Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope (HST). These data were used to construct color-magnitude diagrams (CMDs) of the two GCs and of the M31 halo. Both GCs have old stellar populations similar to those found in Galactic GCs. G302 and G312 have iron abundances of [Fe/H] 5 21.85 5 0.12 and [Fe/H] 5 20.56 5 0.03, respectively. G302 shows no evidence for a color gradient, but the inner regions of G312 are bluer than the outer regions. The halo CMDs indicate that the M31 halo stars have a mean metallicity of with a spread of — [m/H] ∼ 20.6 2 . The shape of the halo [m/H] distribution 2.0 & [m/H] & 20.2 is similar to that in the halo of the Milky Way, but the M31 halo has times that of the Galactic halo. The mean — [m/H] ∼ 8 metallicity of stars in the halo of M31 is ∼4 times greater than that of the M31 GC system, unlike the situation in the halo of the Milky Way, where both the stars and the GC system have similar mean metallicities. Finally, no evidence is found for the presence of young stars, or of a second parameter problem, in either GC. Figure 1 shows the dereddened CMDs for the two GCs and the corresponding halo fields. Structural parameters for G302 and G312 were derived from the WFPC2 images and compared to those measured on the HRCam images. The tidal radii and ellipticities from the CFHT data agree with the more reliable results obtained using the HST data. Using the same methods, core radii are found to agree when the seeing FWHM is &4 times the core radius. Star counts reveal an extended elliptical stellar halo beyond the fitted King-Michie tidal radius of G302. The size of this extended halo is consistent with stellar evaporation rates that have been predicted for Galactic GCs. The orientation of this halo is combined with the GC’s published radial velocity to derive orbital parameters for G302.